The transformation of alicyclic ketoximes into the isomeric lactams according to the Beckmann rearrangement is usually carried out on an industrial scale, such as to form .epsilon.-caprolactam from cyclohexanone oxime, in a homogeneous strongly acid phase by means of, e.g., oleum or sulfur trioxide dissolved in liquid sulfur dioxide. The lactam-laden reaction mixture is then neutralized with ammonia water, after which the acid lactam is separated from the resulting solution of ammonium sulfate. Ammonium sulfate is subsequently recovered by crystallization from the solution that has been freed of lactam.
According to this process the preparation of .epsilon.-caprolactam is also accompanied by the production of a large amount of ammonium sulfate, often of the order of 1.7 to 1.9 tons of ammonium sulfate per ton of lactam if the conversion has been effected in oleum. Such a by-product is commercially undesirable owing to the increasing difficulties in selling ammonium sulfate, so that attempts have been made for some time to find methods of effecting the intramolecular rearrangement of oxime into lactam without by-production of ammonium sulfate.
It has already been proposed to effect this rearrangement in the gaseous phase at a high temperature in the presence of solid acid catalysts, such as boron oxide, but this method is technically and economically less attractive because the process flow, that is gases instead of liquids, occupy relatively large volumes so that the cost of apparatus and the cost of processing are high as compared with a processing in the liquid phase. Furthermore it seems that the high temperature of the process in the gaseous phase is not favorable to the quality of the resulting lactam.
It has also been proposed to effect the rearrangement under the influence of strongly acid cation exchangers in the H.sup.+ form, in which process the ion exchanger comes into contact with oxime dissolved in a solvent. Cation exchangers that are mentioned as suitable for this process are sulphonated copolymers of polystyrene divinyl benzene resins, which are commercially available under the registered trade marks of "Amberlyst 15" from Rohm and Haas and "Dowex 50" from Dow Chemical.
Unlike in the rearrangement of oxime into lactam in a homogeneous medium of oleum or sulfur trioxide in liquid sulfur dioxide in which the lactam formed is liberated by neutralization of the medium with ammonia, a neutralization step of this type is omitted provided a strongly acid cation exchanger is used.
According to the process described in the British Patent Specification 1,029,201, in which cyclohexanone oxime dissolved in a reaction mixture of water-free acetic acid and acetic anhydride comes into contact with a strongly acid cation exchanger, the resulting lactam combines with the ion exchanger. The lactam can then be recovered by separating the ion exchanger and reaction liquor, washing the exchanger with water or a diluted alcohol solution and then evaporating the solvent from the lactam solution obtained. This is a multi-step process with difficulties in product and solvent recovery.
According to the process described in the Japanese Patent Publication 16,777/66, cyclohexanone oxime is brought into contact with a cation exchanger in a reaction mixture of toluene, cyclohexane, and acetic anhydride. Here the resulting lactam does not combine with the ion exchanger but remains in solution. After separation of ion exchanger and reaction liquor, lactam can be recovered from the solution by successively evaporating cyclohexane, toluene, and acetic anhydride.
In these known processes, a water-free organic acid and acid anhydride, usually acetic acid and acetic anhydride, and, in some cases, an organic solvent or mixture of solvents are always used in the reaction mixture in addition to the strongly acid cation exchanger. Depending on the circumstances the resulting lactam may combine with the ion exchanger used.
Attachment of the lactam to the ion exchanger is to be avoided. A process in which the resulting lactam combines with the ion exchanger and can be separated from the ion exchanger by a sodium hydroxide treatment is unattractive for commercialization on an industrial scale owing to the necessity of using a batch-type process; a process in which the resulting lactam remains in solution is much more favorable economically. According to such a process the ion exchanger need not be lixiviated with water to permit the recovery of the lactam and the required amount of ion exchanger will be much smaller than the stoichiometric amount required, that is an amount of 1 mole of lactam per gram equivalent of H.sup.+ of the ion exchanger. In this manner the ion exchanger only serves as a catalyst for the Beckmann rearrangement and does not become loaded with lactam. The lactam that remains in solution can be recovered by distillation, but the comparatively high temperature needed to evaporate the dissolved acetic anhydride adversely affects the quality of the lactam produced.
Attempts have also been made according to Japanese Patent Publication 20,619/66 at effecting the Beckmann rearrangement with a strongly acid cation exchanger for catalyst, the oxime to be transformed being then dissolved in an organic solvent, such as benzene and toluene but, in contrast to the method described above, in the absence of an organic acid or acid anhydride. In such a process hardly any transformation appears to take place at a temperature of about 100.degree. C, and the use of a much higher temperature e.g. above 130.degree. C to promote the transformation is less desirable, since experience has shown that the ion exchanger cannot withstand such high temperatures.
The present invention is directed to an improved process for transforming alicyclic ketoximes, such as cyclohexanone oxime and cyclododecanone oxime, into the corresponding lactams under the influence of strongly acid solid catalyst, in particular strongly acid cation exchangers in the H.sup.+ form, wherein the oxime to be transformed is dissolved in a water-free solvent. According to the disclosed process the presence of an organic acid or acid anhydride in addition to the solid acid catalyst is not needed, thereby reducing the cost of commercialization and operation.
We have found that the Beckmann rearrangement of oximes can profitably be carried out if the oxime is first dissolved in dimethyl sulfoxide (DMSO) and is then contacted with a solid strongly acid catalyst. To obtain a reasonable reaction rate, it is desirable to use a temperature in excess of 85.degree. C but not greater than 130.degree. C, preferably about 100.degree. to about 120.degree. C.
Preferably the dimethyl sulfoxide is water-free or substantially water-free. Molecular sieves may be employed to remove water from the solvent if needed.
The advantage in using dimethyl sulfoxide for the solvent is that the same solvent both activates and regenerates the solid acid catalysts, so that the catalysts can be used several times. Also the transformation of oximes into lactams by means of solid acid catalysts can be carried out by a continuous manner according to the disclosed process. The catalyst is used in a dry state, that is free from or substantially free from water to achieve best results. If the catalyst contains water it may be dried by first washing with anhydrous solvent prior to commencing the process. The catalyst may also be dried by heating.
As used herein the term solid acid catalyst indicates boron oxide or a strong-acid cation exchange resin in the H.sup.+ form. These resins include the sulfonated copolymers of styrene and divinylbenzene, the active group being --C.sub.6 H.sub.4 SO.sub.3
H, as recognized in the art and described in the Encyclopedia of Polymer Science and Technology, Volume 7, pages 692-704, the disclosures of which are hereby incorporated by reference. Preferred ion exchange resins have a total capacity in meg/q of about 5. The following examples refer to "Amberlyst 15," "Amberlite 200" and "Amberlite CG120" resins commercially available from Rohm & Haas Co. and "Zeroliet 227" manufactured by United Water Softeners.
The ion exchange resins disclosed herein are regenerated by treatment with 100 weight percent sulphuric acid, a solution of sulphuric acid in dimethyl sulfoxide or a solution of sulfur trioxide in concentrated sulphuric acid (oleum).
The alicyclic ketoximes to be converted according to the disclosed process have from 3 to 13 carbon atoms as does the corresponding lactam. Preferred and cyclohexanone and cyclododecanone for the commercial importance of their corresponding lactams. For example caprolactam may be polymerized according to known methods to produce nylon-6.
The conversion reaction is conveniently conducted at atmospheric pressures and, although higher and lower pressures may be used, they will generally only add to the cost of equipment, operation or both. The time required for completion or substantial completion of the transformation is subject to wide variation dependent upon the reactants, catalyst surface area and the temperature.